1. Field of the Invention
The present invention relates to a power feeding device and a contactless power feeding system provided with the power feeding device.
2. Description of the Related Art
In recent years, various kinds of electronic devices have been spread and various products have been on the market. In recent years, while spread of a portable electronic device, such as a mobile phone and a digital video camera, is noticeable, a electric vehicle which is driven by electric power, such as an electric car and a hybrid car, begins to appear, on the market.
Such an electronic device and a electric vehicle usually include a battery as a storage means. For now, a power feeding method for charging the battery is direct contact between a household alternating current power supply and the battery.
On the other hand, a power feeding method in which a battery is charged without contact has been researched and developed. Examples of typical methods thereof are an electromagnetic coupling method (also referred to as an electromagnetic induction method), an electric wave method (also referred to as a microwave method), an electromagnetic resonance method (also referred to as an electromagnetic resonance coupling method) and the like.
Note that various contactless power feeding methods utilizing the electromagnetic resonance method are developed so as to increase in electric power transmission efficiency (power transmission efficiency), as shown in Patent Documents 1 to 3. In the case of a contactless power feeding method utilizing the electromagnetic resonance method, the maximum electric power transmission efficiency is not always obtained even when resonance frequencies of a device receiving electric power (hereinafter, a power receiving device) and a device supplying electric power (hereinafter, a power feeding device) are the same as each other. In improvement of transmission efficiency, an important factor is a coupling coefficient of electromagnetic resonance coupling between the power feeding device and the power receiving device, but a too-high coupling coefficient results in decrease in transmission efficiency. The coupling coefficient of electromagnetic resonance coupling changes depending on positions of (the distance between) the power feeding device and the power receiving device. Therefore, in order that electric power is stably-transmitted utilizing a resonant contactless power feeding technique, it is important to control the transmission efficiency to the maximum regardless of the positions of the power feeding device and the power receiving device (the coupling coefficient of electromagnetic resonance coupling).